1. Field of the Invention
The invention relates to floating moored oilfield production structures for deepwater application. More particularly, the present invention relates to the apparatus and a method for the construction and installation of floating oilfield drilling and production structures, wherein the deck structure is supported by multiple pairs of inclined buoyant legs which in turn support a subsurface damping plate structure for reducing the motions of the platform.
2. Description of the Related Art
Suitable deepwater floating production platforms for the offshore oil industry are needed to permit the economical development of petroleum reserves in the increasingly deep waters in which fields are being located. Economic constraints require that the production platform have an efficient design that is installable in a completed condition on location in deep water at an affordable cost. The current platform designs, while adequate in most respects, are sufficiently expensive that many production fields are not developed.
The current state of the art for floating production platforms includes a wide variety of structural types. Semisubmersible structures such as those commonly used for drilling rigs are often utilized. While these semisubmersibles have acceptable motion responses in normal weather, their motion responses during severe storm conditions are typically excessive and unacceptable for some applications.
One production platform structure used is the tension leg platform, which has a hull roughly comparable to a semisubmersible and a vertical array of mooring tendons under high tension. However, this type of structure is quite expensive and its installation is complex. Spars are tubular structures with their axis of symmetry generally vertical. Because of their design, spars must have their deck load of production equipment installed offshore, which is a very expensive procedure.
A new production platform structure that has not been put into service yet is the extendable draft platform (EDP). This type of platform has a hull form generally resembling a semisubmersible, but with multiple large vertical parallel primary legs. The EDP has a damper plate attached to multiple legs at their lower ends. These legs extend upwardly through the deck of the structure. In shallow water, the damper plate is positioned immediately below the hull in order to provide a minimum draft. When the EDP is towed into sufficiently deep water, its damper plate can be lowered on the legs and the tops of the legs attached to the floating deck. The tops of the legs are then deballasted until the air gap between the underside of the deck and the water is sufficient for operating conditions. The EDP has the disadvantage of requiring high lifts of leg components during construction and costs that, while possibly cheaper than a spar, can still be prohibitively expensive except for relatively large production fields.
There is a need for a less expensive deepwater floating production platform with desirable wave motion response patterns, or seakeeping ability. Also, there is a need for a floating platform configuration that is relatively insensitive to vortex induced vibrations (VIV) in currents. There is a further need for a deepwater floating drilling and production platform that can be economically constructed without the use of expensive high lifts and which permits an inshore deck completion and hookup.
There is also a need for a floating platform that can have its major constituents built at a variety of locations and then the platform assembled at any selected location. Additionally, there is a need for an improved drilling and production platform that is easy to transport and erect at a chosen location.
Further, there is a need for a floating platform configuration that can be readily converted from a drilling configuration to a production arrangement. Existing platform configurations require that the platform either be made very large to accommodate both operational phases or that two separate structures be used.